The use of transformers in electrical systems is well known in the art. Transformers are commonly used to provide galvanic isolation for wires carrying signals over substantial lengths, such as wires delivering audio input to loudspeakers. The frequency range over which a conventional transformer is capable of providing linear, distortionless, and un-attenuated signal transfer, however, is limited by the magnetic saturation of the transformer's core. Saturation occurs when a transformer is driven to induce a net flux density higher than its core can support. It is known from transformer theory that flux density is proportional to the ratio of winding current to frequency. Thus a transformer will tend to saturate at higher currents and lower frequencies.
As a high voltage distributed line audio transformer approaches saturation at low frequencies, its impedance decreases rapidly causing an abrupt increase in current draw which can cause the driving amplifier to go into a self-protection mode or fail. A small number of transformers nearing saturation may not pose a problem to a large, well designed amplifier, but as the number of transformers on a given line increases, their combined impedance drop near saturation may appear as a dead short to the amplifier, causing said amplifier to interrupt the program in an effort to prevent its own destruction. Good system design dictates that a high pass filter be inserted into the signal chain at the amplifier input for the purpose of filtering out those low frequencies likely to cause a problem. Presently, the accepted practice when increasing the number of transformer equipped loudspeakers on a high voltage distributed line is to raise the high pass filter frequency to a point that will not allow any frequencies capable of causing transformer saturation to be passed down the line. (E.g., instead of a 50 Hz hi-pass that may be suitable for one or two speakers, a large number of the same speakers may require a hi-pass frequency of 100 Hz or higher to protect the amplifier from the combined effects of transformer saturation.) This increase in hi-pass filter frequency greatly reduces low frequency response and causes music to sound “thin” or “tinny,” which may not be a problem in voice-only applications but is unacceptable in systems designed primarily for music.
In audio applications, for instance, the limitations attributable to core saturation are particularly apparent in the performance of commercially available small transformers at lower frequencies. One known alternative for improved low frequency performance is to use a larger transformer. In applications where space is at a premium, such an alternative is often not a viable one. Moreover, larger transformers are heavier and costlier.
Another alternative is to avoid transformer coupling altogether. Transformerless systems, however, lack the advantages of transformers in large distributed systems such as independent level adjustment of each loudspeaker and the ability to operate at high voltages and proportionally lower line currents thereby reducing line losses and wire size requirements. They are also limited in the number of loudspeakers that may be driven on a single line due to the combined impedance of the loudspeaker load quickly dropping below what a commercially available amplifier is capable of driving. For this reason alone, driving more than four loudspeakers on the same line without transformers is impractical at best.